I'm assuming you have a PCI Express graphics card. Do you have PCIE support activated in your kernel?_________________Kali Ma
Now it's autumn of the aeons
Dance with your sword
Now it's time for the harvest

Sorry, I'm really at a loss here. I've installed/ updated Gentoo on machines with nvidia quite often, but I never ran into problems such as you describe.

What I do is:
Activate PCI Express support in kernel
Deactivate any NVidia Framebuffer related stuff in the kernel
put VIDEO_CARDS="nvidia" (and whatever else I happen to have) into make.conf
emerge xorg-server, nvidia-drivers, etc.
run nvidia-xconfig
startx

As far as I can see you did all this. _________________Kali Ma
Now it's autumn of the aeons
Dance with your sword
Now it's time for the harvest

Sorry, I'm really at a loss here. I've installed/ updated Gentoo on machines with nvidia quite often, but I never ran into problems such as you describe.

What I do is:
Activate PCI Express support in kernel
Deactivate any NVidia Framebuffer related stuff in the kernel
put VIDEO_CARDS="nvidia" (and whatever else I happen to have) into make.conf
emerge xorg-server, nvidia-drivers, etc.
run nvidia-xconfig
startx

As far as I can see you did all this.

The only clue now may be the disappearance of nvidiactl and nvidia0 under /dev.
Or I will reinstall the whole system at this weekend.
Thank you as same

"If you use the proprietary driver from nVidia and the X server fails to start on a udev-only system, then make sure you have the nvidia module listed in /etc/conf.d/modules."
from http://www.gentoo.org/doc/en/udev-guide.xml
but I tested and also doesn't work ....

Sorry missed the hybrid graphics.
As far as I know linux does not have the ability to run hybrid graphics with nvidia video cards, there is a bumblebee project but not successful so far.
I believe there is some reported success with using the nvidia card in cases where the nvidia card can be enabled and the intel card disabled in BIOS.

Do you have a choice(s) in BIOS that enables or disables the intel video card and/or enables or disables the nvidia video card? Do you have a choice that disables switching?